It's a "miracle" no one died in a gang-related shooting that injured 13, including a 3-year-old boy, at a South Side Chicago park Thursday night, Chicago police said today.

The weapon used in the shooting was an assault rifle with a high-capacity magazine, according to police Superintendent Garry McCarthy.

"A military-grade weapon on the streets of Chicago is simply unacceptable," he said at a news conference this morning.

The shooting occurred around 10:15 p.m. local time at a basketball court in Cornell Square Park in the city's Back of the Yards neighborhood. Among the shooting victims was 3-year-old Deonta Howard, who is expected to survive, according to McCarthy and a family representative.

"We're not clear on the intended target at this point, but there were a number of gang members on the scene," McCarthy said, adding, "quite obviously, a 3-year-old is not a gang member."

Deonta is at Mount Sinai Hospital with a gunshot wound to the ear that exited through his mouth, police spokesman Ron Gaines told ABC News.

Four others are in serious condition and the rest are in stable or good condition.

Ages of the victims range from 3 to 41 and include two teenagers, Gaines said. Three of the victims are women.

"It happened so fast," a witness only identified as Jack said. "I was over at the side of the park and I heard the shots. I came over here and there was a lot of people down."

Police superintendent McCarthy said there might have been as many as three shooters involved and authorities were "interviewing a number of people," but no one was in custody.

Mayor Rahm Emanuel canceled events in New Jersey and Washington to travel back to Chicago in the wake of the incident.

The shooting comes nearly three weeks after Chicago saw an outburst of violence Labor Day weekend that ended with eight dead and 20 others injured. The city's Police Department has responded to shootings that have grabbed national headlines by stepping up its crime-fighting efforts, paying overtime to add patrols to some hard-hit neighborhoods, including the Back of the Yards, where Thursday's shooting took place.

McCarthy today noted that authorities have already seized 5,000 guns this year, emphasizing the need to change the gun laws that he said would avoid more deadly shootings.

"It's not just a police problem; there's a structural problem with our laws that facilitates the flow of illegal guns to our streets," he said.

In a February visit to his hometown, President Obama addressed the rising violence. Obama said absent fathers and broken marriages are among the contributing factors in cases of youth poverty, violence and crime. He said one of the solutions is strengthening parental role models.

"There's no more important ingredient for success, nothing that would be more important for us reducing violence than strong, stable families, which means we should do more to promote marriage and encourage fatherhood," Obama said.

South Side teenager Hadiya Pendleton has become a symbol for the tragic loss resulting from gun violence in the city. Pendleton, 15, was gunned down days after performing at Obama's inauguration earlier this year.